There has conventionally known an optical granular material sorting machine that sorts non-defective products and defective products by blowing granular materials such as grains and resin pellets or removes foreign materials or the like mixed into the granular materials by air blowing.
This type of granular material sorting machine sorts the granular materials falling along a predetermined trajectory from an end portion of a conveying path by removing defective products based on a detection signal indicating the defective products or the like by air blowing.
The above granular material sorting machine blows only the defective products or the like by air from among the granular materials falling continuously and in large quantity. In order to blow only the defective products or the like with high accuracy while keeping other granular materials intact, the blowing apparatus needs to include a valve with excellent responsiveness.
In light of this, there has been proposed a piezoelectric valve that can open and close a valve at high speed using a piezoelectric element (see Patent Literature 1).
The piezoelectric valve uses characteristics of the piezoelectric element having excellent high-speed responsiveness and includes a displacement magnification mechanism that magnifies a small displacement of the piezoelectric element based on lever principle.
In the piezoelectric valve, when a voltage is applied to the piezoelectric element, a displacement in an extending direction of the piezoelectric element is transmitted to a valve body via the displacement magnification mechanism and causes the valve body to quickly move and the piezoelectric valve to open.
Then, in the piezoelectric valve, when the voltage application to the piezoelectric element is canceled, a restoring force of the piezoelectric element trying to return to its original state is transmitted to the valve body via the displacement magnification mechanism and causes the valve body to quickly abut against a valve seat and the piezoelectric valve to close.
Accordingly, the blowing apparatus including the piezoelectric valve includes a blowing nozzle incorporating a plurality of piezoelectric valves arranged in parallel with each other; and a compressed air supply device supplying compressed air to the blowing nozzle. A drive unit selectively drives the plurality of piezoelectric valves to blow the compressed air through each nozzle hole of the blowing nozzle.
Moreover, since the responsiveness at the time of opening and closing the piezoelectric valve is markedly superior to that of the conventional electromagnetic valve, the optical granular material sorting machine using the blowing apparatus can blow off the defective products or the like with high precision and is less likely to blow off the non-defective products.
Furthermore, the piezoelectric element has excellent characteristics such as low energy consumption associated with operation, being suitable for high-speed operation, and reduction in size.
However, since the piezoelectric element is vulnerable to a high-humidity environment, the use of the piezoelectric valve in a high-humidity environment may cause an insulation deterioration which is considered to be caused by migration, leading to burnout.